The substitution of aluminum or aluminum-based alloy roof panels for the low-carbon steel or steel alloy roof panels most commonly used in motor vehicles is an attractive option for vehicle mass reduction. Often, however, the remainder of the vehicle body structure continues to be fabricated of steel. Joining an aluminum roof panel to a steel body panel is difficult due to the thermal expansion considerations of the dissimilar materials. The combination of the aluminum roof panel attached to the steel body may create compressive stresses in the aluminum roof panel when the body is subjected to elevated temperatures such as those required to cure or bake the paint applied to the body. These stresses may lead to unacceptable appearance features in the visible segment of the roof panel.
Manufacturers currently secure the aluminum roof panel to the steel body panel after the weld process in assembly. This process typically includes an adhesive bonding operation. Self-piercing rivets can also be used to secure the aluminum roof panel to the steel body panel. This approach, though appealing from a vehicle mass-reduction viewpoint, raises issues due to the significantly different coefficients of thermal expansion of aluminum and steel (about 22.5×10−6 m/m K for aluminum and about 13×10−6 m/m K for steel). The adhesive must be able to absorb the distortion caused by the thermal expansion difference between the roof panel and the steel body panel. Further, because the steel and aluminum are permanently joined together by the rivets, this difference in thermal expansion of steel and aluminum will develop stresses in the aluminum and steel whenever the vehicle body temperature differs from the temperature at which the joint was made. The highest temperature experienced by the vehicle body is during manufacture, when the assembled body is painted. Automotive paint consists of a number of layers, applied separately and then cured at elevated temperature. The paint is cured by passing the painted body through one or more paint bake ovens to raise the body temperature to about 180-200° C. and maintain it at that temperature for at least 20 minutes. This elevated temperature may be sufficient to initiate plastic deformation in the aluminum roof panel. Since plastic deformation is not reversed on cooling, any such deformation may result in an appearance feature such as a crease or buckle in the roof panel which would be unacceptable to the customer.
Further, having the roof and vehicle body panels in contact or electrically connected can create galvanic corrosion. The risk for this corrosion is increased when water is present, such as in the roof gutter areas. As it relates to the adhesive, the current joining process uses the paint bake ovens to cure the adhesive bonding the aluminum roof panel to the steel body panel. However, the heat from the paint bake ovens can cause distortion of the aluminum roof panel relative to the steel body, creating a bowing effect. If left unconstrained, the roof panel would bow enough to break the adhesive bond between the roof panel and vehicle body.